Nicole Cueto sobbed as she locked the door of her apartment and heard her dog, Marty, crying hysterically on the other side.

"He was anxious and upset," the Manhattan resident recalled of the incident earlier this year when she had to go out to work for the first time in months. "It was extremely traumatic for the both of us, and I could not focus on my job for days after."

cute small dog standing on two legs and looking away by the window searching or waiting for his owner. Pets indoors

To help her dog cope, Cueto has invested $340 in professional pet training and counseling — a fraction of the amount many concerned owners are forking over — and Marty is becoming accustomed to being apart from his person. (iStock)

To help him cope, Cueto has invested $340 in professional pet training and counseling — a fraction of the amount many concerned owners are forking over — and Marty is becoming accustomed to being apart from his person.

The boxer mix is among millions of animals across the US who are already suffering from separation anxiety or are set to experience its painful effects after enjoying human company 24/7 since the pandemic took hold in March 2020.

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Now that people are getting vaccinated, offices are re-opening and the work-from-home trend is diminishing, the critters are feeling lonely and confused.

"We’ve been inundated with inquiries from worried dog guardians," expert animal trainer and counselor Malena DeMartini-Price told The Post. "They’re taking little trips out of the house, and their pets are shredding the carpets, and neighbors are complaining about barking.

"All the indicators show something is not well with their dogs."

One of her clients, Jane Yates, has spent $4,800 so far to help her 18-month-old mutt, Jasper, adjust to the sad reality that she is physically returning to work in less than two weeks.

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"We’re paying $800 per month — the cost of a large car repair — but it’s worth every cent," said Yates, a lab administrator in Portland, Oregon, who enlisted the virtual services of a DeMartini-Price associate last October.

"All dogs have to learn that nobody can be truly present for them 24/7/365 and to live with their emotions and discomfort," she added.

The pup, rescued off the streets and believed to be a Chihuahua/pit bull mix, bonded with Yates to such an extent that he once dug holes under a fence to try and follow her out.

"I returned from my walk to hear him alternating between barking and howling from a block away," said his 55-year-old human. "If I so much as put on a mask or picked up my keys, he would jump on the door and cry."

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The solution has been a desensitization program supervised remotely by DeMartini-Price’s employee, Tiffany Lovell, which involves Jasper spending incrementally longer periods of time apart from his owner.

For example, Yates began leaving her kitchen for the garage and returning after a few seconds. Then, when Jasper eventually recognized she would always come back, she increased the minutes of absence. Over the winter, she worked in the garage for several hours using a space heater to keep warm.

This story continues in the New York Post.